feat(admin-console): Stage 6 — mail & notifications domain

Add the mail, notifications, and broadcast pages over the mail, notification,
and diplomail services (no new business logic), completing the operator console.

- GET  /_gm/mail                         deliveries (paginated) + dead-letters
- GET  /_gm/mail/deliveries/{id}         delivery detail + attempts
- POST /_gm/mail/deliveries/{id}/resend  re-enqueue a non-sent delivery
- GET  /_gm/notifications                notifications + dead-letters + malformed
- GET/POST /_gm/broadcast                multi-game admin diplomatic broadcast

Console depends on MailAdmin / NotificationAdmin / DiplomailAdmin interfaces
(satisfied by the concrete services); pages render in tests without a database.
Delivery detail and dead-letters live under /_gm/mail/deliveries/* and
/_gm/mail/... static segments to avoid a param/static route conflict. Resend
and broadcast flow through the CSRF guard.

Tests: mail page, delivery detail (+ not-found), resend (+ bad-CSRF),
notifications overview, broadcast form + send (input assertions) + bad game
ids, and unavailable. Plus an integration test that drives /_gm end to end
through the real gateway → backend (401 challenge + authenticated dashboard).

Docs: backend/docs/admin-console.md page inventory completed.

backend/internal/adminconsole/templates/pages/mail.gohtml

@@ -0,0 +1,32 @@
+{{define "content" -}}
+<h1>Mail</h1>
+<nav class="subnav"><a href="/_gm/mail">Deliveries</a> · <a href="/_gm/notifications">Notifications</a> · <a href="/_gm/broadcast">Broadcast</a></nav>
+{{with .Data}}
+<section class="panel">
+<h2>Deliveries</h2>
+<table class="list">
+<thead><tr><th>Delivery</th><th>Template</th><th>Status</th><th>Attempts</th><th>Next attempt</th><th>Created</th></tr></thead>
+<tbody>
+{{range .Deliveries}}
+<tr><td><a href="/_gm/mail/deliveries/{{.DeliveryID}}"><code>{{.DeliveryID}}</code></a></td><td>{{.Template}}</td><td>{{.Status}}</td><td>{{.Attempts}}</td><td>{{if .NextAttempt}}{{.NextAttempt}}{{else}}—{{end}}</td><td>{{.Created}}</td></tr>
+{{else}}<tr><td colspan="6"><span class="note">no deliveries</span></td></tr>{{end}}
+</tbody>
+</table>
+<nav class="pager">
+{{if .HasPrev}}<a href="/_gm/mail?page={{.PrevPage}}&amp;page_size={{.PageSize}}">&laquo; prev</a>{{end}}
+<span>page {{.Page}} · {{.Total}} total</span>
+{{if .HasNext}}<a href="/_gm/mail?page={{.NextPage}}&amp;page_size={{.PageSize}}">next &raquo;</a>{{end}}
+</nav>
+</section>
+<section class="panel">
+<h2>Dead-letters</h2>
+<table class="list">
+<thead><tr><th>Delivery</th><th>Reason</th><th>Archived</th></tr></thead>
+<tbody>
+{{range .DeadLetters}}<tr><td><a href="/_gm/mail/deliveries/{{.DeliveryID}}"><code>{{.DeliveryID}}</code></a></td><td>{{.Reason}}</td><td>{{.Archived}}</td></tr>
+{{else}}<tr><td colspan="3"><span class="note">no dead-letters</span></td></tr>{{end}}
+</tbody>
+</table>
+</section>
+{{end}}
+{{- end}}